Protective covering for airfoils



May 2 195@ T. c. HARPER 2,505,501

rao'mcnvs covsamc FOR AIRFOILS Filed Feb. 25, 1944 patented May 2, 1 950 UNITED STATES PATENT OFFICE PROTECTIVE COVERING FOR AIRFOILS Thomas C. Harper, Akron, Ohio, assignor to The B. F. Goodrich Company, New York, N. Y., a corporation of New York Application February 25, 1944, Serial No. 523,867

Claims. (Cl. 244-134) This invention relates to protective coverings for airfoils and especially to coverings for preventing the accumulation of ice thereon.

In' prior constructions of protective coverings in which a stretchable portion, between a central inflatable tube zone and an attaching margin, has been subjected to stretch for removing ice from such portion upon the inflation of each tube in said zone, the action of such coverings under some ice conditions of particular severity has not been effective to obtain full cleanliness of ice removal in the stretchable portion. Also, heavy ice caps over the inflatable tubes in the central zone have at times not been broken and removed with the full promptness and thoroughness desired. Very thin ice formations in this zone also have at times been difiicult to remove. The restriction to inflation in the central tube zone by reinforcement, which has been provided therein to transmit the desired stretching force to the stretchable portion rearward of the tubes, has not been conducive to the most effective action in breaking and removing the ice cap. Also, the rapidly repeated stretching of the stretchable portion rearward of the tubes has under some ice conditions tended to craze or merely crack the ice repeatedly rather than break and remove it thoroughly from the surface of the covering.

While the ice conditions which have caused these difllculties are encountered perhaps infrequently, it is desirable that the protective covering be of a construction such as to operate with full effectiveness under all icing conditions. Tothis end, an object of the invention is to provide for breaking and loosening effectively I ice caps on the leading edge of an airfoil thereby facilitating the removal of such ice caps by the ambient flow of air.

. Another object is to provide for thorough removal of the ice from a stretchable portion rearward of the immediate leading edge.

Another object is to provide for uniformity of stretch chordwise with little or no contraction spanwise of the protective covering, especially of stretchable or extensible portions rearward of an inflatable tube zone.

Further objects are to provide for imparting a substantial stretching force to the stretchable portion during inflation of a tube in the tube zone and for exerting little or no stretching force on such portion during inflation of another tube in such zone; and to provide for a condition of low installed stretch or elongation of the covering between the attaching margins thereof, while effectively avoiding objectionable lifting of the covering from the airfoil surface.

Still further objects are to provide for maintaining the desired aerodynamic characteristics of the airfoil, especially during substantial stretching of the stretchable portion so as to obtain the ice-removing benefits of good laminar flow in this action; to provide durability, thinness and simplicity of construction and light- Weight; and to provide for convenience of manufacture and installation and effectiveness of operation.

These and other objects and advantages of the invention will .be apparent from the following description.

In the accompanying drawings which form a part of this specification and in Which like numerals are employed to designate like parts throughout the same:

Fig. 1 is a perspective view of a protective covering or apparatus mounted on the leading edge of an airplane wing, and constructed in ac-v cordance with and embodying the invention, parts being broken away and in section,

Fig. 2 is a plan view from above of the cover: ing before being installed on the wing, parts being broken away,

Fig. 3 is a cross-sectional view of the covering installed on the wing showing two cap-breaking tubes in the inflated condition with little or no stretching of the extensible portions rearwardly of such tubes,

Fig. 4 is a view like Fig. 3 but showing an associated stretching tube in the inflated condition stretching chordwise the rearward extensible portions, and

Fig. 5 is a view in an enlarged scale of a part of the extensible portion showing a kind of highly elastic fabric embedded therein, parts being broken away.

In the illustrative construction shown in Fig. 1, a protective covering In of elastic construction including resilient rubber or other rubber-like material is mounted on the leading edge of a wing H of an aircraft or airplane, the wing having a nacelle l2 housing a motor (not shown). driving a propeller I 3 for propelling the airplane. Other airfoils, struts and other elements of the aircraft may be similarly provided as desired.

The protective covering I 0 is mounted in a stretched condition chordwise of the wing II and secured to the wing skin along upper and lower attaching margins I4 and 45 extending spanwise of the wing. One result of the construction described herein is that a materially lesser degree of stretch in the installed condition is feasible than in prior constructions. The attaching margins M and I may be secured to the wing as by attaching screws l5 extending through upper and lower marginal fairing strips l1, l1 and through the covering It and into the skin of the Wing The attaching margins l4 and I5 may have reinforcing strips or bead elements |8, |8 of stifi material such, for example, as metal Wire included therein.

The covering Iii includes an outer layer or covering of suitable resilient rubber-like material and preferably is of varying thickness, be-

ing relatively thin in section at a tube zone 2|,

for accommodating the different thicknesses of other parts such as extensible or stretchable portions 22 and 23 comprising the protective covering ||l while providing a smooth exterior contour thereby maintaining the desired aerodynamic characteristics of the wing II. By virtue of the resiliency of the rubber material, such layer '20 resists effectively the abrasive action of foreign material in the airstream impinging thereagainst.

One or more and preferably a pair of inflatable cap-breaking tubes 2t, may be individually relatively small in cross-section as compared to an inflatable tube 26 which is larger .in .cross-. section. The tubes 24 and 25 may be in superimposed relation with the tube 26 and disposed beneaththat portion of the outer layer 2|! overlying the tubes 24, 25 and may be positioned .between the layer and the tube 26. The tubes 24, 25 and :26 are disposed in and extend along the tube zone 2|, and the tubes 24 and 25 are positioned in chordwise series with their margins adjacent and preferably contiguous immediately at the frontal portion of the leading edge. A filler strip of suitable resilient rubber-like material may-be superimposed on the adjacent margins of the tubes 24 and 25 beneath the outer layer 20, as shown in Fig. 1, for aiding in obtaining the desired smooth exterior contour of the covering and for providing a strong juncture and anchorage.

The construction hereinabove .described is advantageous in that substantially all the pulling stress chordwise in the covering in the installed condition is taken through the underlying tube '25 rather than through the overlying tubes 24 and 25 whereby the covering remains against the wing with little or no local lifting from the wing at the tube zone during inflation of the tubes 24 and 25. This action makes possible the avoiding of local entrapment of air beneath the cover-. ing at the tube zone 2| and its associatedsubsequent objectionable bulging of the extensible portions 22, 23 because of the flow'of such entrapped air to such portions from the tube zone upon deflation of the tubes 24 and 25.

For inflating the tubes in the desired cyclic manner, connector means 27 in communication with the tubes may be provided at one end of the latter, as shown especially in Figs. 1 and 2. Fluid conduits 28 and 29 are connected to such connector means 2'! and to suitable control apparatus (not shown) and to a source of air under pressure, which apparatus and source of air may be disposed, for example, within the fuselage of the aircraft.

The cap-breaking tubes 24, 25 which are mounted preferably immediately at the leading edge of the airfoil, have walls of relatively great stretchability as compared to that of the associated stretching tube 26. Such walls include in 4 the rubber thereof reinforcing material such as stockinet or other knitted or otherwise stretchable fabric, preferably of nylon or rayon, although cotton or other suitable material may be used. The nylon stockinet fabric is stretchable both circumferentially and longitudinally of the tube, which permits extensive stretching of the tube wall for expanding the covering locally at the tube zone 2| to break and loosen the heavy ice cap.

When the tubes 24 and 25 are inflated first in the cycle of inflation, preferably simultaneously, they tend to stretch outwardly from the wing H and to assume the relatively sharp curvature afiorded by a semi-circular shape in section as shown in Fig. 3, by virtue of the uniform and high stretchability of the Walls thereof. As the tubes 24 and 25 are thus highly distended they operate not only to crack an ice cap upon the leading edge, but also to shift the pieces of the cracked ice cap somewhat in the chordwise direction so as to subject the pieces to the force of the air flow, rather than permit such pieces to remain static in the frontal pressure zone. The outer portions of these cap-breaking tubes offer but little restraint against distension while the inner portions of the tubes are restrained by virtue of the stretched and .tensioned condition existing the installed covering between the attaching margins, the holding stress in such covering at the tube zone being taken largely by the tube 26 beneath tubes Z i and 25. As a consequence of the expanding action of the tubes 24v and25, the surface of the outer layer 20 is stretched locally with low exertion of stretching force upon the rearward extensible portions 22 and 23.

The remaining tube v26, which like the pair of tubes 24, 25 is disposed in the tube zone 2|, and preferably beneath such pair of tubes adjacent the wing ll, may have its opposite margins ex-- tending chordwise beyond the margins of the tubes .24 and 25 for facilitating the anchorage of the extensible portions .22, 23 thereto, as shown especially in Figs. 1 and 2, which anchoring arrangement also promotes freedom of expansion of the tubes 24, '25 and facilitates transmitting the holding stress in the covering to and through the material of such tube 25. The rubber wall of the tube 26 includes textile fabric material, preferably square-woven fabric, straight-laid, for limiting the expansion of the tube 25, such fabric being relatively inextensible as compared to the fabric included in the tubes 24, 25. Such expansion-limiting fabric reinforcement resist effectively the local stretching of the rubber wall and when inflated, the tube 2% assumes an outwardly curved shape of lesser sharpness of curvature than that of the tubes 24, 25 as shownin 'Fig. 4. This action further assists removal of any remaining broken pieces of the ice cap by the air flow and efiectively imparts a relatively great stretching force to the extensible portions22 and 23 and moves the latter portions outwardly from the'wing while maintaining the desired aerodynamic contour at the leading edge and avoiding objectionable turbulent flow of the ambient air across the Wing Because a good aerodynamic shape is made possible, this construction is advantageous in cases where it is desired to use the inflatable structure for purposes of varying the aerodynamic characteristics of the wing, the inflation being such as to make possible a material increase in the wing area with or without a change in the wing camber.

, The extensible or stretchable portions 22, 23

include the overlying portions of the outer rubber layer 20. Also, these portions 22 and 23 include inner rubber layers 3|, 3! adjacent the wing skin and intermediate layers 32, 32.0fhighly elastic sheet fabric material embedded in. and between the innerandfouter layers, as shown especially in Figs. 1 and 5. This highly elastic fabric is stretchable chordwise and this result may be provided by a woven fabric having straight warp threads 32a with adjacent filling threads 32b, 32c highly twisted in reverse directions (Fig. 5), which fabric has extensive stretchability along the filling threads, disposed chordwise, and has little or no stretchability along the warp threads, disposed spanwise. By virtue of such woven construction the elastic fabric does not require the force of the resilient rubber with which it is impregnated and coated for returning to its original condition after being stretched, being in itself highly resilient. Thus, both the rubber and the fabric are better able to resist the development of objectionable permanent set.

The uniformity of stretching afforded by the elastic fabric contributes to effective ice removal by imparting the stretching forces uniformly to and throughout the extent of the outer surface at the extensible portions 22, 23 thereby stretching the material beneath as well as around the bases or seats of the adhering ice particles and loosening the same, which action is not fully available in an unreinforced rubber sheet that tends to transmit a stretching force around an ice particle rather than through its seat. The

elastic fabric is advantageous also in giving added resistance throughout its coverage to objectionable lifting of the covering in any part of its extent. Also, it is not subject to objectionable contraction spanwise of the extensible portions.

For the construction shown front marginal portions of the intermediate fabric layers 32, 32 are overlapped with and are secured as by rubber to the margins of the stretching tube 26 for anchorage thereto to facilitate stretching the extensible portions 22 and 23 when such tube is inflated, with little or no stress on the walls of the cap-breaking tubes 24, 25 and to facilitate transmitting the static holding stress in the covering largely through the material of the tube 26. The elastic fabric extends chordwise to permit folding the fabric around the respective beads [8, I8 at the attaching margins I4 and I5 for holding the beads and receiving the attaching screws [6. An offset arrangement is made possible for a flush mounting of the fairing strips For venting the protective covering 10 to prevent objectionable lifting of portions of such covering under the aerodynamic forces at the leading edge, the inner layers 3|, 3| of the extensible portions 23, 23 may be provided with a multiplicity of spaced-apart rectilinear rib elements 33, 33 extending spanwise along the inner faces thereof for defining venting channels. Venting grooves 34 extending chordwise and being in communication with the channels may have venting apertures 35, 35 therein disposed along lines spanwise at corresponding chordwise positions, which apertures are in communication with the outer air. The venting channels may also be in communication with the outer air at the ends of the protective covering l0. This subject matter of the channels and grooves for venting action is set forth more fully in the copending application of Willson H. Hunter and William C.

Green, Serial No. 482,536, filed April 10, 1943, now Patent No. 2,421,621.

. The fabric and rubber parts of the covering III are united by vulcanization and heat under pressure.

The construction herein described eliminates to a large extent the need for fillets; and eliminates the necessity for special tear-resisting strips, sponge-rubber fillers and special fabric reinforcements at the attaching margins. It also makes possible a thinner, lighter, and more nearly uniform construction. Manufacture is facilitated and a product of improved aerodynamic cleanliness is provided.

Inthe operation of the protective covering "I, it is desirable that the pair of inflatable tubesfl, 25 be inflated first in the cycle and simultaneously by admitting the air under pressure through the conduit 28 and connector means 21. These tubes then distend outwardly to a substantially semicircular shape in cross-section in the manner shown in Fig. 3, the walls of such tubes stretching locally to a relatively great extent within themselves by virtue of the stretchable action of the nylon fabric, and the outer portions of these tubes under inflation spread apart somewhat due to the curvature at the leading edge region, all of which assists in breaking ice caps. While the outer layer 20 at the tube zone 2| is stretched extensively, such distending and stretching action of the tubes 24 and 25 exerts little or no stretching force upon the adjacent extensible portions 22, 23, which portions and the inner part of the covering at the tube zone 2| remain against the wing with negligible lifting therefrom during this phase of the cycle of inflation by virtue of the construction of the covering as described hereinabove. The stretching of the tubes 24 and 25 is found to be highly effective at the immediate leading edge region in cracking and loosening the ice cap even though the ice cap be heavy. However, such inflation need have little or no effect on rearward ice formations on the extensible portions 22 and 23', which ice formations are taken care of during the inflation of the tube 26;

For the removal of the sheet-like ice formar tions which extend rearwardly and adhere to the extensible portions 22, 23, the pair of tubes 24, 25 is deflated and the associated stretching tube 26 is then inflated by air under pressure flowing through the conduit 23 and the connector means 21. Since the wall of the inflatable tube '26 includes the expansion-limiting reinforce ing fabric which is relatively inextensible, such tube 26 distends outwardly and assumes an ex-. panded condition substantially as shown in Fig. 4:. This action is'elfective in imparting a relatively great stretching force to the extensible portions 22, 23 in the chordwise extent substantially uniformly throughout their spanwise extent to break and loosen the ice formations on the portions '22, 23. The highly elastic fabric 32, 32 in such poritions transmits the imparted stretching forces uniformly to the outer layer 20 whereby such outer layer stretches under as well as around the bases of the adhering ice particles. In this man ner the ice on these portions 22, 23 is broken into pieces, which facilitates the thorough and prompt removal of such pieces by the nearly laminar flow of air over these portions of the covering.

The effectiveness of this ice removal in the portions 22, 23 is found to be greater than if these portions were to be stretched more frequently, as for each tube inflation, but to a lesser extent of stretch than is here obtained, and the .im-

p'f'cve d ice-removing action is contributed to also by the greater uniformity of stretchability'of this portion made possible by the elastic vfabric reiniorcement above described.

The construction herein described also permits installing the covering with low percentage of static :stretch between the attaching margins thereby making possible greater dynamic stretching of the stretchable portions than has been feasible in prior constructions, which result is advantageous in promoting the thorough and prompt removal of ice formations on these sportions.

v Still another feature of the protective covering is that, when the stretching tube 26 is in the inflatedcondition, the covering assumes a shape in section such that the gencralairfoil contour at the leading edge is maintained even though the chordwise extent of the airfoil is increased by the movement of the covering outwardly from such leading edge. Maintaining the airfoil contour'promotes smooth flow of air about the coveringand wing during the operation of such coverther'eby maintaining to a high degree the aerodynamic characteristics of the wing.

Variations may be made without departing from-the scope ofthe invention as it is defined "in" the following I claims.

'I- claim':

1-. Apparatus for preventing the accumuiation of ice on the leading edge of an airfoil, said apparatus comprising a resilient covering for said leading edge comprising an expansible acne inicludingan inflatable portion and a second inflatable ipo'r tion outwardly overlying the first said inflatable portion, an extensible portion between said zone and a :margin of the covering, tsaid extensible-portion being stretchable substantially unito'rnrly in the chordwise direction and being substantially inextensible in the spanwise direction, the first said inflatable portion including stretch resistingmaterial and being cons noted and arranged for imparting upon inflation a stretching force to said extensible portion, the said. second inflatable portion including material =01 relatively *high :stretchability and being constructed and arranged for local inflation of said second inflatable portion by virtue of the high stretc hability thereof without exerting substan- Etial stretching force on said extensibleportion. v :2. Apparatus for preventing the accumulation \iceion the leading edge 01 an airfoil, said apparatus comprising a covering including resilient rubber-alike material for said leading edge having a tube none and an extensible aportion between said 'zon'elanda marginof the covering, an inflateible tube in said zone, and a pair of inflatable tubes arranged in series chordwise and superimposed on the first said tube in said zone, ad- I-jacent margins of said pair of tubes being ,a-nchored to the first said tube for resisting outward movementof said adjacent margins relative to said'leading edge upon inflation of said pair of tubes, the first said tube having an expans'ion limiting reinforcement therein efiecfive-upon inflation thereof to impart a stretching force'to said extensible portion, and said pair of inflatable tubes having walls of relatively great 'sti etchabilityas compared to the first said tube for local stretchingupon inflation thereof with aelatively low exertion upon said extensible poritlonof stretching force as comparedto the first said tube and-with minimum outward movement of said adjacent ma'rgins.

4'3.- flipparatusfor preventing the accumulation v 8 of ice on 'thesurraceof an airioil, said apparatus comprising a covering of resilient rubbers-like materialand an elastic fabric embedded in said material and continuous and substantially co.- extensive withithe dimension ofsaid covering in one direction, said fabric comprising warp ele: ments extending in said one direction of the covering and filling :elementsdisposed crosswise thereof, adjacent filling elements being highly twisted "in reverse directions whereby said fabric is substantially inextensible in said one direction and highly stretchable crosswise thereof and is substantially noncontractible in said one direcr tion under the crosswise stretching.

4. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil, said apparatus comprising a protective covering in,- clu'ding resilient rubber-like material for said leading edge having a'tube zoneextending span? who of the covering rand an extensible portion extending spamvise between said zone and a margin of the covering, said extensible portion comprising sheet elastic fabric material substantially coextensive with the spanwise dimension or said covering, said fabric material having relatively greatstretchability in the chordwise direce ;tionrand being relatively inextensiole in the. spam wise direction, an inflatable tube in said zone having a stretch-resisting fabric material :re-

inforc'ement therein effective upon inflation of the tube to impart a stretching force to said extensible portion, a second inflatable tube in said .zone outwardly overlying and anchored to the first said inflatable tube, said secondinflat-nable tube having a wall including stretchable fabric material of relatively great :stretchability .:as compared to the first said tube ,for local stretching circumferentially'upon inflation of the second tube with relatively low exertion of stretching force ascompared to the firstsaid tube upon said extensible portion, means for attach;- ing said-covering at said margin, and means for inflating the tubes.

5. Apparatus for preventing the accumulation of ii'ce'on the :leading edge of an airfoil, said an,- lparatus comprising-a covering of resilient rubber:- lilce :material Iorsaid leading edge having s paced? apart extensible portions including attaching margins and a tube zone intermediate said extensible eportions, said extensible portions com.- prising elastic fabric substantially stretchable in the 'chordwise direction and relatively inextensible in the spanwise direction, an inflatable tube of substantially inextensible fabric materialzin said tube Zone beneath :said covering effective upon inflation to impart stretching force to said extensible portions, a pair of adjacent inflatable tubes in said tube zone beneath said covering superimposed on said inflatable tube, the pair of inflatable tubes comprising fabric material hav- :ing relatively great stretchability as compared to the fabric material of the first said tube for local stretching upon-inflation thereof with relatively low exertion of stretching force as cornpared to the first said tube upon said extensible portions, means for attaching said covering at said margins, and means for inflating the tubes.

6 Apparatus for preventing the accumulation :of ice on "the leading edge-of an airfoil, said ap; paratus comprising :a covering including iresilient -mbber-like material for said leading :edge, said covering comprising an expansibleazone extending in one direction of :said covering which ex,- pansible zone includes an in'flatable portiomsaid covering comprising also :an extensible "portion extending in said direction along said zone between said zone and a margin of the covering and including substantially all-over sheet elastic fabric material continuous and substantially coextensive with the dimension of said covering in said direction, said fabric material comprising substantially non-stretchable elements of filamentary material extending in said direction and stretchable elements of filamentary material extending crosswisev of said direction so as to be stretchable crosswise of said direction and substantially noncontractible in said direction under thecrosswise stretching, and means for attaching said covering at said margin.

'7. Apparatus for preventing the accumulation of ice On the leading edge of an airfoil, said apparatus comprising a covering of resilientruba her-like material for said leading edge, said covering comprising an inflatable tube having strechresisting fabric material therein extending in one direction of said covering, a second inflatable tube of fabric material of relatively high stretchability as compared to. that of said stretch-resistsing fabric material extending in said one direc-- tion of the covering and outwardly overlying the first saidinflatable. tube, and an extensible portion extending in said direction along said tubes between said tubes and a margin of said covering, said extensible portion including substantially all-over sheet elastic fabric material. substantially coextensive with the dimension of said covering in said direction and continuously secured along one margin of said extensible portion to the first said inflatable tube, said sheet elastic fabric material comprising warp elements extending in said direction of the covering and filling elements disposed crosswise of said direction, adjacent filling elements being highly twisted in reverse directions, said sheet elastic fabric material being stretchable crosswise of said direction and substantially noncontractible in said direction under the crosswise stretching, and means for inflating said tubes whereby the first said tube under inflation is effective to impart substantially uniform stretching force to said extensible portion substantially throughout the extent of said portion.

a 8. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil, said apparatus comprising a resilient covering for said leading edge comprising a spanwise-extending expansible portion including an inflatablepassage and a second inflatable passage outwardly overlying the first said passage, and a spanwiseextending extensible portion including substantially all-over sheet elastic fabric material therein between said expansibe portion and a margin of the covering, said sheet elastic fabric material. being stretchable substantially uniformly in the chordwise direction and being substantially inextensible in the spanwise direction, the first said inflatable passage having a stretchresisting reinforcement therein effective upon inflation of the passage to impart relatively great stretching force to said extensible portion, and said second inflatable passage having an outer wall including material of relatively high stretchability as compared to the first said inflatable passage for local inflation of said second inflatable passage by virtue of the high stretchability of said outer wall without exerting substantial stretching force on said extensible portion.

9. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil. said apparatus comprising a resilient covering for said 16 leading edge comprising an exp ansible zone including an inflatable portion and a second inflat able portion outwardly overlying the first said inflatable portion, and an extensible portion be-. tween said zone and a margin of the covering,.

the first said inflatable portion including stretch-- resisting material and being constructed and ar-. rangedfor imparting upon inflation a stretching force to said extensible portion, the said second inflatable portion including material of relatively high stretchability as compared to said stretch! resisting materialv and being constructed and ar-. ranged for local inflation of said second inflatable portion by virtue of the .high stretchability there-J of without exerting substantialstretching force. on said extensibleportion.

10. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil, said ap.- paratus comprising a resilient covering for said leading edge-comprising an expansible zone in-v cluding an inflatable tube and a second inflatable tube outwardly overlying the first said inflatable tube, and an extensible portion extending along substantially the entire length of said tubes be-,

tween said zone and a margin of the covering, said-i extensible portion" being stretchable substantially uniformly in thedirection toward said margin. throughout said length, the first said inflatable tube including stretch-resisting material in the wall thereof and being constructed and arranged.

for imparting upon inflation a stretching force. to said extensible portion, the said second infiat-.v

able tube including material in the wall thereof of relatively high stretch abiltiy .as compared to that of said stretch-resisting material and being 6011-.

structed and arranged for local inflation of said.

second inflatable tube by virtue of the high stretchabilit thereof without exerting substan-- tial stretch-ing force on said extensible portion.

11. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil, said apparatus comprising a resilient covering for said leading edge comprising a spanwise-extending expansible Zone including an inflatable tube and. a second inflatable tube outwardly overlying the first said inflatable tube, and a spanwise-extending extensible portion between said zone and a margin of the covering, the first said inflatable tube including stretch-resisting fabric material and being constructed and arranged for imparting upon inflation a stretching force to said extensible portion, the said second inflatable tube including fabric material of relatively high stretchability circumferentially of such tube as compared to that of said stretch-resisting fabric material and being constructed and arranged for local inflation and circumferential stretching of the outer wall of said second inflatable tube by virtue of the high stretchability thereof without exerting substantial stretching force on said extensible portion.

12. Apparatus for preventing the accumulation of ice on the leading edge of an airfoil, said apparatus comprising a resilient covering for said leading edge, said covering comprising an expansible zone including an inflatable tube extending in one direction of said covering and a second inflatable tube extending in said direction and outwardly overlying the first said inflatable tube, and an extensible portion extending in said direction along said zone between said zone and a margin of the covering. the first said inflatable tube, including stretch-resisting fabric material in the wall thereof and being constructed and arranged for imparting upon inflation a stretching force to said. extensible portion, the second, said inflatable tube including fabric material in the wall thereofv of relatively high stretchability as compared to that of said stretch-resisting fabric. material and. being constructed and arranged for local inflation. of said. second inflatable tube by virtue of the high stretchability thereof without exerting substantial stretching force, on said extensible portion, and said extensibleportion including substantially all-over sheetv elastic fabric material substantially coextensive with the extent. of? said tubes in said, direction, said sheet elastic fabric material comprising warp elements extending in. said direction of. the covering, and

filling elements disposed crosswise of said direction, adjacent filling elements being highly twisted in reversedirections whereby said. sheet elastic fabricmaterial is highly stretchable crosswise of.

said direction of the. covering and substantially noncontractible in said directionunderthe cross.-

wise stretching.

l3, Apparatus for preventing the accumulation ofice on the leading edge of an. airfoil, said. an paratus comprising a. covering. of resilient rubberelilse material for said leading edge having a.

tube zone and an extensible portion between said zone and a. margin of the covering, means for at taching said covering at said margin, an infiat able tube of stretch-resisting fabric material in said zone, and. a. pair of inflatable tubes each relatively small as compared to'the first said inflatable tube extending. in side-by side relation between the adjacent. margins of the first said inflatable tube. in said zone and outwardly. overlying the first said inflatable tube, the first said inflatablev tube being constructed and arranged for imparting upon inflation. a stretching force to; said extensible portion, and said pair of infiat-able tubes. including fabric material in the walls thereof having relatively great stretchabiiity as: compared to that ofsaid stretch-resisting fabric material and. being constructed and arranged for local inflation of said pair of inflatable tubes by virtue of the high, stretchability thereofwithout exerting substantial stretching force on said extensible portion.

14. Apparatus for preventing the accumulation of ice on the leading edge. of an airfoil, said apparatus comprising a protective covering including resilient rubber-like material for said leading edge, said covering having an expansible portion extending in the spanwise direction and an, extensible portion extending in the said directionbetween said expansible portionanda spanwise extending margin of the. covering, said ex.- tensible portion comprising. substantially all-over sheet elastic. fabric. material having spanwiseextending substantially non-stretchable elements of filamentary material and chordwisesextending stretchable elements: of filamentary material so as to have relatively great stretchability in. the chordwise. direction. and. be substantially noncontiactible in the spanwise direction under the. chordwise stretching, said sheet elastic fabricv material being continuous. and substantially coextensive with the spanwise dimension of. said covering, means forattaching said covering at.

saidmargin, and means for expanding said ex pansible portion and stretching said extensible: portion.

15. Apparatus, for preventing the accumulation of ice on the leading edge of an airfoil, saidv apparatus; comprising an elastic covering including resilient rubber-like material for said leading edge; saidcoverlng comprising; an underlying portion, of substantially inextensible textile; fabricextending spanwise and an underlying extensible portion of substantially all-over sheet. elastic fabric secured'to and extending chordwisefrom. the-first said underlying; portion to an atl taching margin, of said extensiblev portion spaced REFERENCES CITED The fol-lowing references are of record in file of this patent:

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